The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image areas. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image areas accept the ink and repel the water. The ink on the image areas is then transferred to the surface of a material in which the image is to be reproduced, such as paper or cloth.
Negative-working lithographic printing plates are prepared from negative-working radiation-sensitive compositions that are formed from polymers that crosslink in radiation-exposed areas. A developing solution is used to remove the unexposed areas of the plates to thereby form a negative image.
The most widely used type of negative-working lithographic printing plates comprises a layer of a radiation-sensitive composition applied to an aluminum substrate and commonly includes a subbing layer or interlayer to control the bonding of the radiation-sensitive layer to the substrate. The aluminum substrate is typically provided with an anodized coating formed by anodically oxidizing the aluminum in an aqueous electrolyte solution.
It is well known to prepare negative-working lithographic printing plates using a radiation-sensitive composition that includes a photocrosslinkable polymer containing the photosensitive group, --CH.dbd.CH--CO-- as an integral part of the polymer backbone see for example, U.S. Pat. No. 3,030,208 (Schellenberg et al), U.S. Pat. No. 3,622,320 (Allen), U.S. Pat. No. 3,702,765 (Laakso) and U.S. Pat. No. 3,929,489 (Arcesi et al)!. A very common commercially useful photocrosslinkable polymer in lithographic printing plates is a polyester prepared from diethyl p-phenylenediacrylate and 1,4-bis(.beta.-hydroxyethoxy)-cyclohexane.
Polyesters that are especially useful in the preparation of lithographic printing plates are those which incorporate ionic moieties derived from monomers such as dimethyl-3,3'-(sodioimino)disulfonyl!dibenzoate and dimethyl-5-sodiosulfoisophthalate. Polyesters of this type are well known and are described, for example, in U.S. Pat. No. 3,929,489 (Arcesi). One polyester of this type is poly1,4-cyclohexylene -bis(oxyethylene)-p-phenylenediacrylate!-co-3,3'-(sodioimino)disulfonyl!d ibenzoate. Another is poly1,4-cyclohexylene-bis(oxyethylene)-p-phenylenediacrylate!-co-3,3'-(s odioimino)disulfonyl!dibenzoate-co-3-hydroxyisophthalate.
Various other photocrosslinkable polymers are known for use in lithographic printing plates including the polyimide precursors described in U.S. Pat. No. 4,416,973 (Goff). Such compositions are known for durability, chemical resistance, adhesion to metals and high mechanical strength when photocrosslinked. Well-known sensitizers including coumarins and halogenated triazines, as described for example in U.S. Pat. No. 4,505,793 (Tamoto et al), can also be used in photocrosslinkable compositions.
It is also known to include a polymerizable vinyl monomer in the photosensitive composition to crosslink the photopolymerizable polymers. Various acrylates and methacrylates are known for this purpose. The literature suggests that either di- or triacrylates can be used for this purpose.
EP-A-0 601 203 (Chisso) describes the use of a composition composed of a polyimide, an acrylated isocyanurate and a polyalkylene glycol diacrylate to allegedly provide improved photosensitivity. However, the pre-imidized polyimide resins described in this reference lack crosslinking functionality. They dissolve only in high boiling organic solvents that are expensive and inconvenient for large-scale manufacturing processes. The only use suggested for some polyimide compositions is as insulating layers in electronic materials. Thus, any graphic arts use of such polymers is missing.
Modern commercial printing plates often require a blend of polymers in the photosensitive layer. Unfortunately, some of those required polymers are incompatible with each other or insoluble in coating solvents, thus necessitating multilayer coatings. Thus, there is a need for a photosensitive formulation that can be used to provide a single-layer element. There is also a need to have a photosensitive formulation that has high photosensitivity and wearability (or durability, that is, can be used for long printing runs).